Cruise Engine Load Percent

My Cats are rated at 2800 WOT and generally cruise at 2100 to 2300 rpms....
75 to 80% is what I have been told also...

Yes, 80% is what most engine manufacturers recommend. I prefer to cruise at 1950 rpm's usually if the engine top rpm is 2350rpms. This usually ends up being less then 80% load and somewhere in the low to mid 70%. But it really depends on the boat and cruise speed.

It depends on what the engine is, and what it is rated for. There is no "One size fits all" number you can use even with the same engine type as they come in various duty ratings. With more specific information as to engine & boat, better advice can be given. For basics though, if you are dealing with high HP light weight applications, figure 65%. Moderate HP light weight, 80%. Low HP, heavy weight (displacement speed boats) 100%.

I'd always learned aprox. 80%. However in recent years several manufacturers have been suggesting 90% which just seems a bit much for my liking. I wouldn't do 100% (except in short spurts to blow it out) on anything but a 2 stroke outboard.

So if you had a CAT 3406 in an "A" rating putting out 425hp at 100% you wouldn't run it at 100%? If so what power would you run a 3406 with an "E" rating that puts out 725hp?

I think NYCAP is referring to engines generally found in yachts which would generally be an "E" rating in most of them. You should cruise at 80% load with an E rating, and can do 100% for no more then an hour at a time. A rating can run at 100% all day long.

Most of the engine manufacturers give, in addition to a recommended cruise RPM, a percent fuel burn for each engine rating. For E and D ratings, typical yacht engines, the fuel burn is rated at 33% to 50% of max fuel burn at full load. It is the fuel burn that the engine maufacturers base the engine longevity and suitability of use. So if one spends most of the time at full crusie speed and very little time at hull speed, then 80% load would far exceed the engine manufacturers recommendations.

I think you really need to look at the recommended % of max RPM and carefully monitor your hours and fuel burn over a season or at least many tank fulls of fuel to evaluate the average fuel burn and load relative to the manufacturers recommendations. We tend to cruise longer distances and so to approach the manufacturers specs we run at near to 70 % load. With our CATs I look at the optimum lbs/hp-hr fuel burn (sweet spot) based on the engine spec sheet combined with when the boat feels nicely on plane to optimize fuel burn, engine load and engine longevity.

If you only consider high speed planing hull boats to be yachts, you would be correct. That however is not the actual circumstance. Many yachts are fitted with high duty cycle and continuous duty engines. That's why I said the question requires greater specificity to the circumstances of what boat/engine combination is being considered. There is no "one size fits all" answer.

Hi,

This information comes from CATERPILLAR it might or might not be any use for what the OP is looking at but as many of the users here run CAT Equipment I thought it was worthwhile posting it.

Ratings Guidelines

Ratings should be applied on the basis of vessel operation, and on the type boat or hull design. Vessel descriptions such as workboat, pilot boat, or ferry boat; and hull design such as displacement or planing hull do not define the operating conditions of the vessel or the power demands on the engine. More than one rating could apply to these descriptions, depending on how the vessel is operated. Listed below are descriptions of ratings A, B, C, D and E followed by an overview chart.

Unrestricted Continuous - A Ratings

For vessels operating at rated load and rated speed up to 100% of the time without interruption or load cycling (80% to 100% load factor).

Typical applications could include but are not limited to vessels such as freighters, tugboats, bottom drag trawlers, or deep river tugboats. Typical operation ranges from 5000 to 8000 hours per year.

For 3600 Engines only:

* Continuous Service (CS) Rating is suitable for continuous duty applications, including dredges, for operation without interruption or load cycling.
* Maximum Continuous (MC) Rating is generally used for vessel applications involving varying loads. The engine power actually produced is limited by application guidelines, leaving a power reserve for unusual operating conditions. Operating time at loads above the Continuous Service rating for a given RPM is limited to 1 hour in 12 or 8.3% of total operating hours.

Heavy Duty - B Ratings

For vessels operating at rated load and rated speed up to 80% of the time with some load cycling (40% to 80% load factor).

Typical applications could include but are not limited to vessels such as mid-water trawlers, purse seiner, crew and supply boats, ferries, or towboats. Typical operation ranges from 3000 to 5000 hours per year.

* Some 3500 engines have "Bhp ratings" which are typically used in fast commercial and passenger vessels including catamarans and SWATH vessels with high load factors.
* 3600 engines indicating "Fast Commercial Vessel Ratings (FCVR)" are typically used in high speed commercial vessel applications with approximate load profile: 85% of operating hours at rated power, 15% of hours at less than 50% rated power.

Maximum Continuous - C Ratings

For vessels operating at rated load and rated speed up to 50% of the time with cyclical load and speed (20% to 80% load factor).

Typical applications could include but are not limited to vessels such as ferries, harbor tugs, fishing boats, offshore service boats, displacement hull yachts, or short trip costal freighters. Typical operation ranges from 2000 to 4000 hours per year.>

* Some 3500 engines have "Chp ratings" which are used in fast commercial and passenger vessels and cruising yachts with moderate load factors.

Intermittent Duty - D Ratings

For vessels operating at rated load and rated speed up to 16% of the time (up to 50% load factor).

Typical applications could include but are not limited to vessels such as offshore patrol boats, customs boats, police boats, some fishing, fireboats, or harbor tugs. Typical operation ranges from 1000 to 3000 hours per year.

* Some 3500 engines have "Dhp ratings" which are used in patrol craft, fast passenger vessels, and cruising yachts with low load factors.
* 3600 engines indicating a "Military Fast Vessel Rating (MFVR)" are typically used in patrol craft and other naval vessels with low load factors.

High Performance - E Ratings

For vessels operating at rated load and rated speed up to 8% of the time (up to 30% load factor).

Typical applications could include but are not limited to vessels such as pleasure craft, harbor patrol boats, harbor master boats, some fishing or patrol boats. Typical operation ranges from 250 to 1000 hours per year.

* Some 3500 engines have "Ehp ratings" which are typically used in fast patrol craft and fast yachts with low load factors.

Rating Conditions for 3500 and smaller engines

Ratings are based on SAE J1228 standard conditions of 29.61 in Hg (100 kPa) and 77° F (25° C). These ratings also apply at ISO3046/1. ISO8665, DIN6271, and BS5514 conditions of 29.61 in Hg (100 kPa), 81° F (27°) and 60% relative humidity.

Fuel rates are based on fuel oil of 35° API [60° F (16° C)] gravity having an LHV of 18 390 Btu/lb (42 780 kJ/kg) when used at 85° F (29° C) and 60% relative humidity.>

Fuel rates are based on fuel oil of 35° API [60° F (16° C)] gravity having an LHV of 18 390 Btu/lb (42 780 kJ/kg) when used at 85° F (29° C) and weighing 7.001 lbs/U.S. gal (838.9 g/liter).

Rating Conditions for 3600 Engines

Ratings are based on SAE J1349 standard conditions of 29.61 in Hg (100 kPa) and 77° F (25°C). These ratings also apply at ISO3046/1, ISO8665, DIN6271 and BS5514 standard reference conditions. Ratings also meet classification society maximum temperature requirements of 113°F (45° C) temperature to turbo and 90° F (32° C) seawater temperature without derate.

Fuel consumption is based on ISO3046/1 with +5% tolerance at rated power for fuel having an LHV of 18,390 Btu/lb (42,780 kJ/kg) and weighing 7.001 lbs./U.S. gal. (838.9 g/liter). Includes engine mounted fresh water and lube oil pumps. SFOC without pumps 2% less.

Additional ratings may be available for specific customer requirements. Consult your Caterpillar® representative for details.

Performance Data

Performance along a typical fixed pitch propeller curve with a 3.0 exponent.

Power rated in accordance with NMMA procedure as crankshaft power. For units equipped with Caterpillar supplied marine gears, reduce crankshaft power by 3% for propeller shaft power.

All Cat Marine engines shipped on or after January 1, 2000 have ratings which meet IMO standards for nitrogen oxide emission limits in Regulation 13 of Annex VI of Marpol 73/78.

Engine Load Percent

With Cat 3406E's rated 800hp. Max rpm is 2320 at 100% load. 2075 rpm is the most efficient planning speed (gallons/mile) at 75% load and 90% rpm. Is it okay to operate at 90% or when planning should they be run at 80%.

E rated CATS are recommended to be run at 80% LOAD, not by RPM, provided they turn the maximum rated RPM at full throttle.

OK, how about the same question for CAT 3406 C's rated at 580hp?

How about: Maximum Continuous - C Ratings

For vessels operating at rated load and rated speed up to 50% of the time with cyclical load and speed (20% to 80% load factor).

Typical applications could include but are not limited to vessels such as ferries, harbor tugs, fishing boats, offshore service boats, displacement hull yachts, or short trip costal freighters. Typical operation ranges from 2000 to 4000 hours per year.>

Thanks for the info.

Same here. Thank you very much.

I've been having this exact argument with Cummins engineers for the last week. I was always under the impression that 80% load was your cruise, at 80% I'm cruising at 32 knots. Cummins insists that 90% load is what they consider cruise for their pleasure rated engines good for 500 hours per year, at 90% my cruise is a little better than 34 knots.

My question is since public perception is that 80% load is cruise, should I consider 80% load my cruise speed or 90% as the engine manufacturer states?

When we puchased our VSC last year, I looked into this in detail for MAN, Volvo and CATS, the three engine choices in the various models we were considering. You can call the engine dealers and they will give a recommended cruise load and typical RPM. But in the written literature from each manufacturer the E and D rated engines are specifically rated as a percent of maximum load fuel burn, 30% to 50% of maximum fuel burn for E and D.

Yes the cruise speed may be 80 or 90% of full load but only if the total average fuel burn per year is limited to the rated 30 to 50% of max fuel burn. I was surprised by this and really came to understand this on the boatdiesel web site and ultimately verified this with the CAT and MAN manufacturers. The price to pay will be shortened service life and potentially premature failures.

The fuel burn is always below 50% for yachts as they always do a lot of idling and slow speed and etc. if you run at 80% load. Cat and MAN recommend 80% load for most all motors, and volvo usually recommends 90%